You would need a new flywheel, clutch, clutch hydraulic system and pedal, 5 speed 2.5 box, possibly different propshaft. The mercedes manual gearchange at that time was not particularly well regarded for its action anyway. Ironically an auto box version of the 2.5-16v may be worth more when it comes to sell since its not so common. Probably not worth destroying the cars originality which is paramount to perceived retained value.